This invention relates to electrostatographic reproduction machines, and more particularly to a hybrid development type electrostatographic reproduction machine having a wrong-sign or wrong-polarity toner purging mode.
Generally, the process of electrostatographic copying is executed by first using a corona generating or charging device to uniformly charge a photoreceptive member to a first polarity, and then exposing a light image of an original document, positioned in registration on a platen, onto the charged photoreceptive member. Exposing the charged photoreceptive member to a light image selectively discharges the photoconductive surface thereof in areas corresponding, for example, to non-image areas in the original document, while maintaining the charge (of the first polarity) on the image areas, thus creating an electrostatic latent image of the first polarity on the photoreceptive member. The undischarged areas comprising the electrostatic latent image are subsequently developed with correct-sign or correct-polarity charged toner particles into a visible toner image. The sign or polarity of such correct-sign or correct-polarity toner, as is well known, is relatively opposite the first polarity of the latent image being developed. Ordinarily, toner particles are contained in the sump of a development apparatus where they are moved and mixed with carrier particles in order to triboelectrically charge the toner to the correct polarity.
The toner image is thereafter transferred from the photoreceptive member onto a clean copy sheet on which the image is then fused or permanently affixed in order to provide a hard copy reproduction of the original document.
Unfortunately, the quality of the development step and that of the hard copy reproduction can be detrimentally affected by the effects of wrong-sign or wrong-polarity toner which forms and accumulates inside the sump of the development apparatus.
Wrong-sign toner in the development sump of an electrostatographic reproduction machine is ordinarily the source of many machine performance failure modes, some of which can have catastrophic effects.
Ordinarily, the generation of wrong-sign toner is not a major problem in electrostatographic reproduction machines using development systems that include a magnetic brush. This is because magnetic brush development systems each have a built-in method of purging themselves of wrong-sign toner. This built-in method involves the magnetic brush which has oppositely charged carrier particles on it that contact both image and background areas on the photoreceptor of the machine. During such contact, wrong-sign toner particles on the magnetic brush see each background area of the photoreceptor as a "development field" to which to transfer. This is because background areas are biased to repel correct-sign toner thus acting as a background cleaning field for toner of the correct sign or polarity. As such wrong-sign toner particles which have a polarity opposite to that of the correct-sign toner particles, obviously become attracted to these background areas, and so are transferred thus from and out of the developer housing.
On the other hand, in hybrid development systems, which have a donor roll and do not have a magnetic brush that contacts both the image and background areas of the photoreceptor, this built-in method has been removed, and there is therefore no built-in normal pathway for wrong-sign toner to be transferred from the development housing. Wrong-sign toner therefore tends to detrimentally accumulate in title development housing.
It is therefore an object of the present invention to provide a method to purge wrong-sign toner from Hybrid Development housings.
It is also an object of the present invention to purge wrong-sign toner according to a process that is transparent to a customer.
It is also an object of the present invention to selectively apply a cleaning field to the roll nip between a magnetic roll and a donor roll of a hybrid development system for cleaning the donor roll of right-sign toner while also attracting wrong-sign toner to it.
It is a further object of the present invention to selectively leave the wires, the SED electrodes, or jumping AC bias means in a hybrid development housing activated such that a wrong-sign toner cloud is created within the development zone.
It is still a further object of the present invention to selectively bias the donor roll and the photoreceptor so that the wrong-sign toner cloud in the development zone will be attracted to the photoreceptor for subsequent cleaning and removal by a cleaning apparatus.